The present invention relates to an ink jet printing apparatus which includes a nozzle for ejecting an ink under supersonic vibration, a charging electrode located in a position where the jet of ink separates into droplets so as to selectively charge the droplets and a deflection electrode deflecting charged ink droplets to cause them to impinge on a sheet of paper. More particularly, the present invention is concerned with an ink jet printing apparatus of the type which employs a gutter as an electrode for detecting deposition of a charge on an ink droplet.
An ink jet printer of the type described has a pump which pumps an ink from an ink reservoir through a filter into an accumulator adapted to smooth the input ink pressure. The accumulator supplies the ink under pressure to an ink jet head which imparts a predetermined frequency of vibration to the ink with an electrostrictive vibrator incorporated therein. The ink under the vibration is ejected from a nozzle of the head. At a position spaced a given distance from the nozzle, the jet of ink is separated into droplets at regular intervals. The period of such separation is equal to the frequency of the vibration generated by the vibrator. A charging electrode is located in a position where the jet of ink separates into droplets. The charging electrode is supplied with a charging voltage whose level varies in steps; which is zero level (e.g. ground level) during a non-printing period (when a video signal is logical "0"). The charging voltage must be fed in a manner of pulses while each step of charging voltage must be impressed in conformity to a certain phase of formation of an ink droplet. To meet these requirements, a phase search is performed to determine the phase of charging voltage pulses relative to that of vibration by the vibrator.
For a phase search, clock pulses are supplied from a clock pulse generator to a drive voltage generator so as to produce a sinusoidal wave synchronous with the clock. The sinusoidal wave is coupled to the electrostrictive vibrator inside the head. The output clock of the clock pulse generator is also supplied to a phase setting circuit to prepare charging clock of a predetermined duration and different in phase from the clock by a predetermined amount. Phase search charging pulses exactly common in phase to the charging clock, identical or opposite in polarity to the charging voltage and having a constant level at all the time are generated by a search signal generator and coupled to the charging electrode via gate and an amplifier. The charge detecting electrode determines whether an ink droplet has been charged. As a charge detection circuit produces a "charge" output before a predetermined number of ink droplets are formed, the phase searching operation is terminated; otherwise, a 1-step phase shift command is fed to the phase setting circuit to shift the charging clock by a given phase relative to the preceding charging clock.
After a proper charging phase of the charging clock has been set up relative to the output clock of the clock pulse generator, a charging signal prepared by a charge signal generator based on the charging clock and having a stepwise level is fed through a gate and an amplifier to the charging electrode. Then, the printer starts its operation in a data reproduction mode. Ink droplets are deposited with charges corresponding to charging voltage levels and are individually deflected by a deflecting electrode in accordance with their specific charges. When the video signal is "0" level, the charging voltage is made zero level whereby ink droplets are collected by the gutter without being charged at all.
Apart from known charge detection electrodes of the cylindrical or U-shaped electrostatic induction type or the type having two flat electrodes adjacent to or opposite to each other, a gutter for collecting ink droplets is usable as such an electrode as disclosed in Japanese Layed Open Patent Application nos. 49-107142/1974 and 55-84680/1980, for example.
In an ink jet printer using a gutter as its charge detecting electrode, the gutter is electrically insulated from ink which has flown downward therefrom into an ink collection system. The gutter is connected with a charge integrating capacitor and a capacitor discharging switch. A voltage charged in the capacitor is amplified and compared with a reference voltage during the phase search mode. A rise of the capacitor voltage beyond a predetermined level within a given period of time indicates that the ink droplets are charged. However, an insulator which supports the insulated gutter becomes spattered with the ink impinged on the gutter resulting in a leak betwen the gutter and the collected ink. This renders the capacitor voltage and, therefore, the charge detection unstable. Particularly, when the spattering or smearing is significant, even a proper charge tends to be identified as a zero charge. Such a problem cannot be settled unless the gutter and its neighborhood is always cleaned at the sacrifice of time and labor. Additionally, due to a frictional charge on ink during ejection from the nozzle and a charge generated by ink upon impingement on the gutter, "charge" signal components unrelated with charging/non-charging of ink droplets by the voltage fed to the charging electrode are introduced into the capacitor as a dc bias to thereby deteriorate the S/N ratio. This unavoidably invites an error unless a larger number of charged droplets are directed to the gutter.
Smearing with ink is the problem also encountered in the case with the electrostatic induction type charge detecting electrode. To prevent introduction of noise, a disproportionately intricate measure is required for processing signals or cutting off noise. This type of detection electrode, in particular, suffers from a drawback that it has to be located somewhere between the charging electrode and the gutter, which increases the distance between the nozzle and the paper sheet. As a result, an ink droplet has to fly a longer distance and, thus, tends to be noticeably dislocated on the paper sheet due to the air resistance, mutual repulsion or attraction between flying ink droplets, charge distortion etc.
The ink fed to the head is usually held at the ground potential, and so is regarded the ink which is wetting the gutter because it is continuous with the ink in the collection system. However, in practice, the resistance of the ink is substantial and the ink in the gutter is sucked by a pump into a collector tank, whereby the ink is discontinued within a collection pipe between the pump and the gutter to increase the gutter-ground resistance to a significant degree. Non-charged ink droplets impinging on the gutter have been charged by the friction at the nozzle or the induction by the adjacent charged droplets, though not positively charged by the charging voltage. The non-charged ink droplets, therefore, shifts the potential on the gutter off the ground level, which in turn disturbs the deflection of charged ink droplets for printing data.